God is regarded as the necessary being. The universe cannot be the necessary first cause because it itself is contingent and not eternal.
No, "Ian" is the necessary being. Ian is eternal and needs no first cause, Ian created the universe. Think I am wrong? Disprove it!
Therefore, the first cause must 'transcend' the universe. That's why people have claimed that this 'first cause' is God, and more specifically, their respective god.
Why must it transcend the universe? How do you know the universe isn't eternal? The "big bang" wasn't the start of the universe, it was the start of the universe in its current state, the universe existed prior to its expansion. There could be much more outside of our expansion, lots of big expanding bubbles which disperse and then have their masses gather together to later form new expansions. If this is all happening outside of our sensory ability (too far away for example) we might never know about it.
That's just one hypothesis, there are many others, "God" is merely the oldest hypothesis - but that's because it is the easiest to explain.
A long time ago:
Question: "How big rock get up mountain?"
Answer: "Big man must have lift rock high!"
More recently:
Question: How did those large rocks get all the way up there?
Answer: The Earth goes through cycles of ice ages, a rock could easily be carried along by a massive sliding sheet of ice and get caught at the top of the hill, the ice sheets then later melt as the planet warms.
But putting aside the attempt to show there is no proof of God's existence. Even IF there is a God, religion claims to know what God wants. Without evidence of God there is clearly no evidence that anyone knows what this god wants, otherwise there would be evidence of God.
Religion is arrogance. It is merely a way to comfort those scared of their own mortality that if they sprinkle some magic water in the right places, eat the "right" food, and say the right magic words they will continue to live forever. The Egyptians made it into an industry with their "Book of the dead" which explained all the magic words and actions you needed to "pass the test" after you die and go to ever green fields to live with your deceased loved ones. Islam is merely one of many mutations on the same theme.